Cotton (Gossypium hirsutum, and to a lesser extent Gossypium barbadense together with other Gossypium species) provides about 55% of the fibre used in textile manufacturing globally and is an important contributor to world economies. The cotton fibre is probably the most elongated cell in the plant kingdom. The molecular mechanisms that control the differentiation of this elongated plant cell are still largely unknown. Although commonly called fibres, these cells are not part of the vascular tissue and arise, instead, from the ovule epidermis. Fibres of cotton are extremely long single elongated epidermal cells that develop on the outer surface of cotton ovules, reaching upwards of 5 centimeters in some species. Fibre initiation starts between a day before and up to a day after anthesis and the fibre initials begin to elongate immediately after fertilisation, ballooning out from the surface of the seed coat epidermis. After a period of elongation, secondary cell wall thickening fills the fibre with cellulose and the fibre dies and collapses to form the mature fibre that is harvested from the seeds.
In contrast to the discovery of numerous genes responsible for fibre elongation and secondary cell wall synthesis, few genes have been identified that are associated with fibre initiation. Early cytological studies showed structural changes in fibre initials occur up to three days before anthesis: including enlarged nucleoli and nuclei, as well as an increased number of Golgi complexes (Berlin, 1986). It has been proposed that the Arabidopsis leaf trichomes, which require at least twenty genes for normal development (Hülskamp et al., 1994), could serve as a model for elucidating the genetic mechanisms controlling cotton fibre initiation and differentiation.
One of the first genes to be characterised in controlling leaf trichomes, GLABROUS1 (GL1), encodes a member of the Myb family of transcription factors (Oppenheimer et al., 1991). An exhaustive search of a cotton ovule cDNA library recovered six novel Myb-domain genes, but none of them encoded a GL1 homolog (Loguercio et al., 1999). Arabidopsis trichome initiation is proposed to be controlled by a trichome promoting complex comprised of GL1, TRANSPARENT TESTA GLABRA1 (TTG1, a WD40 protein), and GLABRA3 (GL3, a basic Helix-Loop-Helix protein). GLABRA2 (GL2, a Homeodomain protein) regulates trichome morphology and spacing and TRIPTYCHON (TRY, a Myb-like protein) mediates lateral inhibition of trichome development in cells adjacent to each trichome (Rerie et al., 1994; Walker et al., 1999; Szymanski et al., 2000; Schellmann et al., 2002; Ohashi et al., 2002). However, genes with similar functions in cotton have yet to be identified, and hence it remains speculative whether these two single celled epidermal hair systems share any common features.
There is a need for the identification and characterization of genes involved in fibre initiation in fibre producing plants such as cotton. This will enable markers to be used to screen plants for desirable fibre traits, as well as allow for the production of transgenic plants with altered fibre production.